1. Field of the Invention
The present invention relates generally to precoat filters such as disc filters having a layer of precoat thereon and, more particularly, to the washing and renewal of a precoat layer on such filters.
2. State of the Art
Filtration equipment such as disc or drum filters are known in the art for their use in separating solid particles from a liquid in which they are suspended. Such filters typically pass the particle-containing liquid through a selected filter media which allows the passage of liquid therethrough while retaining the particles on a surface of the filter media for subsequent collection. The liquid passing through the filter media is typically referred to as filtrate while the particles on the surface of the filter media are typically referred to as cake.
The filter media used in such a process may be formed from a variety of different materials but is often of a cloth- or fabric-type material, including, for example, wire mesh, formed of natural or synthetic materials. Additionally, filter media may include a built-up layer of cake on the filtration surface of a drum, disc, or other filter element. This layer of cake is referred to as a precoat. It is noted that, depending on the type of material being processed, the precoat layer may be formed from a material that is the same as or similar to the material being filtered. For example, if lime mud is being filtered and removed from a liquid, lime mud may also be used as the precoat. However, the precoat does not have to be formed of the same material as that which is being filtered.
The precoat layer is typically formed on top of a filter cloth, with the two elements working in conjunction to separate filtrate from the process cake. The precoat layer is a permeable layer which allows filtrate to pass through while capturing additional particles on a surface of the precoat layer. The filtered particles are typically removed, often by a scraping device, while the precoat layer remains on the filter media for continued filtering.
After a precoat layer has been in use for an extended amount of time, the precoat layer becomes less permeable and thus reduces the efficiency of the filtering operation by allowing less filtrate to pass therethrough. When the precoat layer is in such a condition, it becomes desirable to remove and regenerate the precoat layer by removing the existing precoat layer of material in its entirety and forming a new precoat layer in its place.
One type of apparatus used in removing and regenerating precoat layers on a filter is disclosed in U.S. Pat. No. 5,759,397 to Larsson et al. (issued Jun. 2, 1998), the disclosure of which is hereby incorporated herein by reference. FIGS. 1 and 2 hereof are representative of the apparatus disclosed in the Larsson patent. FIGS. 1 and 2 show a disc filter 10 including a plurality of hollow discs 12 having sidewalls 14 of a filter material 16. The hollow discs 12 are coupled to a hollow axle 18 and are in fluid communication therewith via holes 20 formed along the hollow axle 18.
The hollow discs 12 are positioned within a container 22, such as a tank, which includes an inlet 24 for introducing a slurry 26 (i.e., a particle-containing liquid) into the container 22. A vacuum 28 draws the filtrate through filter material 16 with particles from the drawn liquid accumulating on the surface of the filter material 16 to form a cake.
As shown in FIG. 2, the container 22 is filled slightly less than halfway with the slurry. The discs 12 are configured to rotate counterclockwise through the slurry 26. In operation, a portion of the discs 12 enters into the slurry 26 to draw filtrate from the slurry 26 and then emerge with a cake formed of accumulated particles. A scraper 30 is used to remove the accumulated cake from the discs 12 while leaving a specified thickness of precoat material. The cake removed from the discs 12 is collected in a chute or bin 32 and transferred from the filter 10 for further processing or disposal, as the case may be.
When it is desired to remove and regenerate the precoat, an oscillating spray pipe 34 having a spray nozzle 36 coupled thereto moves between the periphery and center of the disc 12 removing the precoat layer by spraying the precoat layer with a pressurized fluid. The Larsson patent describes such removal and regeneration as taking place in two steps with half of the discs being stripped of the their precoat layer followed by replacement of such layer, while the precoat layers of the other half of the discs are not sprayed.
It is believed that such an arrangement may expose too much surface area of the fabric filter media by removal of the precoat layer therefrom, thus resulting in the loss of vacuum crucial to operation of the filter. If the vacuum is broken, it is likely that the precoat layers remaining on the additional discs (i.e., those not having the precoat layers renewed) will drop their precoat layers, causing a serious and expensive interruption in the operation of the filter.
In addition to potential loss of vacuum, removal of the precoat layer using high pressure fluid may cause damage to the underlying filter media, particularly over time. Thus, while it may be desirable to remove and regenerate precoat at fairly frequent intervals, such frequency may necessitate a consequent undesirable frequency of downtime of the filter system for replacement of the filter media.
U.S. Pat. No. 5,897,788 to Ketolainen et al. (issued Apr. 27, 1999) discloses the use of a spray nozzle oriented at an acute angle to the surface of a filter drum of a drum-type filter to remove an outer, more porous and softer portion of the precoat layer on a substantially continuous basis between removals of the precoat layer in its entirety down to the filter wire.
Other filters have likewise used swinging spray showers to assist in washing of process materials on precoat layers such as that disclosed in U.S. Pat. No. 6,063,294 to Martensson et al. (issued May 16, 2000), the disclosure of which is hereby incorporated herein by reference. The Martensson patent discloses a spray assembly having multiple nozzles in each spray pipe and wherein the speed at which the spray pipe traverses the face of a disc is varied according to its radial position relative to the disc. An additional feature disclosed in the Martensson patent is an independent washing shower which is optionally used to soak process material on the exterior of the precoat and thus increase the efficiency of cake removal using the scraper. In essence, the washing shower includes a spray pipe and one or more nozzles directed at the filtering surface of a given filter (i.e., the filtering surface of a drum or a disc). Using a disc as an example, and considering the disc to be divided into a plurality of segments, the wash assembly sprays water on the precoat surface of a given disc segment subsequent to the disc segment being rotated through the pool of slurry. The wash water flows through the filter media, including the precoat, forcing any filtrate potentially remaining in the cake and precoat layers to be drawn through the filter by the vacuum. Without the washing apparatus, some filtrate liquid might potentially be discharged with the cake layer, thus causing some inefficiency with the operation.
While the washing shower provides some increased efficiency in production of filtrate, such shower assemblies require piping and valving in addition to that required for the precoat removal system. Additionally, the inclusion of a washing shower may add constraints to the overall design of the filter as the shower heads take up additional space and must be designed so as to not interfere with any moving components of the filter.
Accordingly, it would be advantageous to provide a precoat removal apparatus and system which allows for maximum efficiency in replacing the precoat layer on filter media of filter discs of a disc-type filter without losing the vacuum (or pressure differential) formed across the filter media. Additionally, it would be advantageous to provide a precoat removal apparatus and system which also incorporates a precoat renewal feature in a disc-type filter to minimize the necessity to remove and regenerate the precoat layer and to potentially reduce the frequency of repair of the filter media.
The present invention comprises a precoat renewal system for a filter such as a disc filter. The precoat renewal system includes a plurality of nozzles coupled to spray piping and valving with each nozzle being positioned for spraying an area of one side of a disc. The nozzles and at least a portion of the spray piping may be movable such that the nozzles may be displaced relative to the discs. The nozzles and piping may be arranged in groups or zones with each zone covering a specified percentage of the discs in the filter. Each zone may represent about 40% or less of the total surface area of the filter media of the plurality of filter discs in the filter. It may be preferred that each zone be configured to effect fluid impingement on between about 15 and 25% of the total surface area of the filter media of the plurality of filter discs.
Each zone may be configured such that high pressure fluid may be supplied to that particular zone for removal of a precoat layer on the disc. The precoat renewal system may be configured so that other zones may be concurrently washed with a lower pressure fluid. The zones may be sequentially or otherwise segmentally provided with high pressure fluid until the precoat layers of each respective zone have been removed and renewed.
A disc-type filter system incorporating the precoat renewal system as well as a method of treating a precoat layer on a disc-type filter including a plurality of filter discs are also encompassed by the present invention.